This invention generally relates to personal gas monitors intended to be worn on the person of anyone likely to be exposed to an atmosphere polluted with noxious gases in concentrations ranging from a fraction to several hundred ppm (parts per million), as typically found in certain industrial situations, and in particular to longitudinally extending personal gas monitors (hereinafter also referred to as personal monitors) containing a column of adsorbent to which the pollutant gains access by molecular diffusion during the period the personal monitor is actually in use and from which it may be thermally desorbed and analyzed later in order to establish the total exposure suffered by the wearer over the said period.
Ideally, a personal monitor must be so constructed as to ensure high repeatability of results, in the sense that the total exposure evaluated by analysis should only be a function of the concentration of the adsorbed gas at the end of a given exposure period. It is largely with a view to securing good repeatability that known personal gas monitor tubes are normally associated with a small vacuum pump for aspiring the polluted atmosphere through a suitable adsorbent, which occupies most of the tube inner volume. The pump is driven by an electric motor powered by a battery. In the present state of the art, pumped tubes systems may be regarded as being generally satisfactory, but they are expensive, comparatively bulky and, therefore, inconvenient in use; above all, they need to be calibrated quite frequently since there are many variable factors, including mechanical wear, that can affect the pumping efficiency and, therefore, the volume of polluted air aspired during a given period. For these reasons non-reusable personal monitor badges relying on molecular diffusion of the pollutant through the adsorbent are in much more common use in all but the most exacting situations.
A badge-type personal monitor is usually constructed in a synthetic resin and may comprise injection moulded parts which snap together. It is comparatively cheap, therefore. In addition, it requires no ancillaries of any kind and contains no moving parts.
Despite claims to the contrary, badge-type personal monitors do not in general provide outstanding repeatability. This may be due partly to the manner of their construction, which does not allow sufficiently narrow tolerances to be maintained, and partly to the undue emphasis placed on sensitivity when the requirements of high sensitivity (and therefore high molecular diffusion rate) and repeatability are largely incompatible. This latter point requires amplification.
It is by now well established that in order to provide good repeatability the rate of diffusion should not be significantly affected by the velocity of the air movement over the diffusion aperture through which the polluted atmosphere gains access to the adsorbent within. It is known that the provision of a stagnant diffusion zone between the said aperture and the free surface of the adsorbent is essential to securing comparative insensitivity to air current. In this the assumption is made that the adsorbent is so efficient that at the free surface thereof the concentration of the pollutant is near zero, and that, therefore, a molecular diffusion gradient tends to be set up in the diffusion zone that essentially varies only in response to concentration changes of the pollutant in the atmosphere. Assuming the personal monitor to be generally cylindrical, the approximation to this desirable condition becomes closer the greater the length of the diffusion zone compared with its average diameter. A factor close to 2 is very desirable. This is not achieved by the badge monitors. In fact, the badges in common use either provide no diffusion zone at all or at best a shallow one. At all events, the length of the zone is not even as great as its average diameter (or, in the case of either circular or non-circular cross-section, its average breadth). If badges were constructed for optimum repeatability, they would have to be unacceptably protruberant when attached to the wearer's clothing. The inadequacy of the diffusion zone is to some extent alleviated in certain designs by the provision of a membrane pervious to the pollutant for the purpose of creating a stagnant barrier layer co-operating with the stagnant atmosphere in the diffusion zone. These designs appear to improve matters in some respects but their repeatability still leaves a lot to be desired, particularly in the monitoring of gas so noxious that only a concentration of a fraction of 1 ppm can be tolerated in an eight-hour exposure period.
Apart from indifferent repeatability, there is another drawback associated with the badges: the adsorbent therein can only be desorbed in the solvent mode-the temperature associated with the alternative thermal desorption mode would cause the badges to collapse. This could be overcome by constructing the badges in a suitable material but then their shape would be unsuitable and would make them quite expensive if realized in a material such as stainless steel, which stands up to the desorption temperature and is comparatively inert.
Solvent desorption involves a comparatively simple manual operation but is much less suited to automation than the thermal desorption mode. In addition, the solvent commonly used is carbon disulphide, which being toxic and very highly inflammable may be more objectionable than the pollutant to be monitored. Having particular regard to the growing concern for the health of industrial workers, likely to result in the passing of legislation compelling employers to carry out mass monitoring of certain categories of workers, the time may soon be here when only an automated system for handling, desorbing and analysing the samples collected through personal monitors will be a practical proposition in large industrial organizations employing thousands of workers potentially exposed to varying concentrations of noxious gases.
It was at first believed that non-pumped molecular diffusion tubes would almost inevitably provide better repeatability compared with badge monitors, since it is so much easier to provide a stagnant diffusion zone having the desired characteristics for repeatability within a longitudinally extending hollow member rather than a shallow vessel. When batches of tubes constructed along conventional lines, which essentially involved locating a column of granular adsorbent between two glass-wool plugs, leaving however a generous air space between the open end of the tube acting as the diffusion aperture and the free surface of the facing glass-wool plug, it was found that repeatability was no better than could be achieved with some known badge monitors. This unexpected finding seemed to suggest that, although the importance of providing an adequate diffusion zone ahead of the adsorbent had been generally appreciated, some unidentified problem stood in the way of obtaining superior repeatability.
In the present context, personal monitors of a given design provide good repeatability if after a statistically significant batch (say, 50) of such monitors has been exposed to a standard polluted atmosphere, desorbed and analyzed, in accordance with a strictly controlled procedure, they yield pollutant concentrations in which the standard deviation is within limits that are tolerable for the chosen pollutant.